Entanglement Coupled Cluster Theory: Exact Spin-Adaptation

TL;DR

This paper introduces a novel spin-adapted coupled cluster method using entanglement with a non-interacting bath to accurately model open-shell molecules, enabling exact spin adaptation and potential extensions to various spin states.

Contribution

It presents a new entanglement-based approach to spin-adapted coupled cluster theory that allows for exact spin adaptation in open-shell systems.

Findings

Proof-of-concept calculations for doublet states demonstrate the method's validity.

The approach can be extended to other total spin states.

The method maintains the accuracy of standard closed-shell coupled cluster theory.

Abstract

We present a novel approach to spin-adapted coupled cluster theory. This approach is based on the entanglement of an open-shell molecule with electrons in a non-interacting bath; together they form a closed-shell state. For the total system, electron correlation can be added with the standard spin-adapted closed-shell coupled cluster formalism. A projection operator, which enforces conditions on the electrons in the bath, is used to obtain the desired state of the molecular system. This entanglement coupled cluster theory is detailed and proof-of-concept calculations for doublet states are reported. The approach is extendable to open-shell systems with other values of the total spin.